Many electronic devices are powered by batteries. Rechargeable batteries are often used to avoid the cost of replacing conventional dry-cell batteries and to conserve precious resources. However, recharging batteries with conventional rechargeable battery chargers requires access to an alternating current (AC) power outlet, which is sometimes not available or not convenient. It would, therefore, be desirable to derive power for electronics wirelessly.
In the field of wireless charging, safe and reliable use within a business or home environment is of the utmost concern. To date, wireless charging has been limited to magnetic or inductive charging based solutions. Unfortunately, these solutions require a wireless power transmission system and a receiver to be in relatively close proximity to one another. Wireless power transmission at larger distances requires more advanced mechanisms such as, for example, transmission via radio frequency (RF) signals, ultrasonic transmissions, laser powering, to name a few, each of which presents a number of unique hurdles to commercial success.
The most viable systems to date utilize power transmission via RF. However, in the context of RF transmission within a residence, commercial building, or other habited environment, there are many reasons to limit the RF exposure levels of the transmitted signals. Consequently, power delivery is constrained to relatively low power levels (typically on the order of milliWatts). Due to this low energy transfer rate, it is imperative that the system is efficient.
In a free space wireless environment, radiation from an omnidirectional radiator or antenna propagates as an expanding sphere. The power density is reduced as the surface area of the sphere increases in the ratio of 1/r2, where r is the radius of the sphere. This type of radiator is often referred to as isotropic, with an omnidirectional radiation pattern, and it is usual to refer to antennas in terms of their directivity vs. gain as dBi—decibels over isotropic. If the intended receiver of the transmission is at a particular point relative to the transmitting radiator, being able to direct the power toward an intended receiver means that more power will be available at the receiving system for a given distance than would have been the case if the power had been omnidirectional radiated. This concept of directivity is very important because it improves the system performance. A very simple analog is seen in the use of a small lamp to provide light and the effect of directing the energy using a reflector or lens to make a flashlight where the power is used to illuminate a preferred region at the expense of having little to no illumination elsewhere.
Central to mechanisms for directionally focusing transmissions in charging-over-the-air (COTA) systems is the ability to switch between receive and transmit modes in order to listen for beacon signals from clients and to provide power signals to clients, respectively. In COTA systems that rely on counters to provide the switching, the transmitter has no real knowledge of which clients beaconed when. In this manner, the transmitter simply returns power to whichever client beaconed when it was listening. Such systems are time-aware but lack the ability to also be client-aware.
Accordingly, a need exists for technology that overcomes the problem demonstrated above, as well as one that provides additional benefits, such as battery charging from received RF power signals while operating. The examples provided herein of some prior or related systems and their associated limitations are intended to be illustrative and not exclusive. Other limitations of existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description.